Conveyor lines typically perform a wide range of functions in modern manufacturing facilities ranging from simple product delivery systems to integral components of manufacturing processes. At the high end, conveyor lines are tasked with precisely positioning pallets or workpieces at automated manufacturing stations located along the conveyor line. The workpieces are usually fixed to the pallets which are then conveyed to workstations disposed along the conveyor line. However, workpieces themselves may also be conveyed along the conveyor line when feasible.
Workstations are manned by human operators, automatic mechanisms, robots or combinations thereof which perform various manufacturing steps on the workpiece. When a manufacturing workstation is occupied by a human operator rather than automatic or robotic equipment, it is not as critical that a pallet, or workpiece, stop at a precise designated point along the workstation because the operator can adapt to slight positional variances. However, when a manufacturing workstation is comprised of automatic or robotic equipment, or the like, it is critical to the manufacturing operation that the pallet be accurately positioned at a designated location within the workstation prior to performing the desired step because automated manufacturing equipment cannot be made responsive to positional variances without increasing the cost and complexity of such equipment.
To this end, power driven conveyor line systems have been developed which satisfy the automated workstation positioning requirements by strictly controlling the linear position of a pallet or workpiece.
It is known in powered conveyors to stop a pallet (or a workpiece) at a designated stopping point by means of electro-mechanical, hydraulic or pneumatically operated stops which are disposed at specific locations along the conveyor line. These stops prevent pallets from moving forward in the direction of conveyor travel while the remainder of the conveyor line continues to operate. A feature of this continuous line operation is that multiple workstations are permitted to function independently from one another while sharing the same conveyor drive. However, such stops suffer significant impact and abuse, and such impact is also imposed on the load.
Another disadvantage of continuous power driven conveyors is that each power driven roller typically incorporates a "slip" clutch which permits the roller to slip while supporting a stationary pallet. Such slip mechanisms add to the cost and complexity of continuous power driven conveyors. Continuous power driven conveyors also typically operate at relatively low speeds, on the order of 45-60 feet per minute, because of the high impact forces imparted on the stops when pallets are driven into them at any higher rate of speed.
Still another disadvantage of continuous power driven conveyors is that "idle" stations must be provided between each of the workstations along the conveyor line to reduce the time required to transfer pallets into and out of the workstations. Generally, with line speeds at 45-60 feet per minute, it may take up to 10 seconds or more to transfer a pallet from one workstation to another workstation (workstations are typically about six to ten feet apart). The "idle" stations reduce the time required to transfer pallets by queuing a pallet at the entrance of each workstation. This undesirably increases the number of pallets and the cost of the system.
It is also known in the conveyor arts to stop a pallet at a designated stopping point on a conveyor line by means of a servo motor control system such as disclosed in U.S. Pat. No. 4,891,568 issued to Shibata et al. Servo motor control systems provide highly accurate positioning characteristics but with higher incremental costs associated with the necessarily complex control circuitry and positioning equipment.
It is also known in the conveyor arts to control pallets within a control zone via rollers, each driven by separate motors, and a number of sensors, such as limit or proximity switches, positioned along the control zone, as disclosed in U.S. Pat. No. 5,086,910 issued to Terpstra. The use of sensors positioned at the upstream and downstream ends of a zone, and the use of individual roller motors, undesirably increases the cost and complexity of the system.
Accordingly, it is therefore an object of the present invention to provide an improved open loop control system for use with a power driven conveyor system which, in a preferred embodiment, eliminates the need for stops or clutch slipping mechanisms within power driven rollers, and which permits individual powered conveyor sections or zones to be started and stopped while accurately stopping a load at a precise location.
It is another object of the present invention to provide an improved control system for use with a power driven conveyor system which, in a preferred embodiment, permits a conveyor line to be operated at higher line speeds thereby eliminating the need for "idle" positions on a conveyor line.
It is yet another object of the present invention to provide a low cost, highly reliable control system for use with a power driven conveyor system.
Other objects and purpose of the invention will be apparent upon reading the following specification and inspecting the accompanying drawings.
In the described embodiment of this invention, there is provided a control system for a power driven conveyor zone which conveys a pallet, or the like, down a conveyor section having a plurality of conveyor zones adjoined end to end. Each conveyor zone includes conveying elements such as rollers, chains or belts traversing a support frame; a single drive motor for driving the driven elements of the zone; a transmission for coupling an output of the motor to the driven elements; and a station located adjacent the zone. The control system includes: a local controller for controlling a plurality of conveyor zones via a number of control signals, and individual conveyor zone controllers for controlling each respective conveyor zone. The zone controller controls the operation of the conveyor zone in response to the control signals. A first sensor is coupled to the zone controller for signalling when the leading edge of the pallet has entered or is approaching the station position. The zone controller initiates a ramp-down control signal in response to the first sensor signal, and the motor responds to the ramp-down signal by decelerating the driven elements. A second sensor is coupled to the zone controller for signalling when the leading edge of the pallet has reached a designated stopping point. The zone controller, in response to the second sensor signal, de-energizes the motor, and applies D.C. injection braking current to the motor thus stopping the pallet at the designated point.